The 31-year-old actress looked the part as the sported a light blue shift dress, bobbed wig and fake baby bump on the set of new mini-series 'The Kennedys', where she will star alongside Greg Kinnear, who plays tragic president John F. Kennedy.

A source said: "It's uncanny how much Katie looked like Jackie. I did a double take."

The eight-hour mini-series is due to air on The History Channel in 2011, and will look at the years in the run-up to the late president's assassination in 1963.

Despite the impressive cast - which also includes Tom Wilkinson as Ambassador Joe Kennedy Sr. - the show has received some criticism, with many claiming the storyline focuses on the family's sexual antics rather than the political story.

However, bosses on the network are convinced Katie's portrayal will be a success.

Executive Nancy Dubuc said: "It's a role that Katie's demure elegance will bring to life."